All optical lenses present some degree of longitudinal chromatic aberration, meaning that the focusing distance for a given lens is at least slightly different for different wavelengths of light. In imaging systems, this means that the image produced is either in-focus or out-of-focus depending on the wavelength or wavelengths of the incoming light. Additionally, some portions of the image may be in-focus while other portions of the image may be out of focus. Also, the image may be geometrically deformed to some degree due to the differences in focal lengths of different wavelengths. These problems become exacerbated as images include a broader spectrum of wavelengths.
For a simple lens, there may be only a single wavelength for which the lens is in-focus at a given focal distance. Lenses with some amount of chromatic correction, such as achromatic lenses, apochromatic lenses, and superachromatic lenses, are designed to reduce the focusing error (e.g., the differences between the focal lengths of the wavelengths for the lens), in a certain spectrum around the desired wavelengths. These chromatically corrected lenses reduce the focusing error in a certain band of desired wavelengths, such as in the visible spectrum. Broad-spectrum lenses—those that correct for focal lengths outside of the visible spectrum—are complicated, heavy, and expensive.